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Armyworm Moth Capture Reaches Record Number

May 2, 2008

In recent days the number of armyworm moths captured in the University of Kentucky-Integrated Pest Management pheromone trap system has set record levels. This could set up the potential for large enough numbers of armyworms to emerge in the weeks ahead to cause crop damage.

For the week ending Friday, April 25, capture of moths in the Princeton trap increased to 600 moths per trap-week, said Doug Johnson, UK extension entomologist. This is a larger peak than in any first generation flight since UK-IPM has been keeping records, and a substantially larger number than in the outbreak years of 2001 and 2006.

Capture in the UK-IPM traps in Lexington for the same week also drastically increased. In fact, the increase over the previous week is much larger in Lexington, from 27 to 660 moths per trap-week, than in Princeton, which was up from 322 to 600 moths per trap-week, though Princeton has the larger total number captured over time.

“It is difficult to know what this means for central Kentucky as we do not have a historic set of data,” Johnson said. “We only have last year with which to compare. Certainly the current 2008 captures are much larger than the first generation in 2007.”

Johnson said the high counts are no reason to overreact.

“This is an early warning, not a prediction of catastrophe,” he said. “I can not tell you if the moth flight will result in a large population of caterpillars. There are many other factors, particularly disease, predation and parasitism that impact how well the eggs and caterpillars will survive. Nevertheless, I do think that we are at an increased risk of economically important infestations compared to most years. Certainly you should be scouting your grass crops.”

Armyworms, or true armyworms as this species is commonly called, occasionally occur in large enough numbers in the springtime to cause damage to pastures, hay, corn and small grains. Armyworms generally appear in Kentucky in mid-May through June.

Cool, wet spring weather usually favors armyworm development. The insect gets its name from its behavior of migrating through fields in large numbers, similar to invading armies.

A degree-day model can be used to estimate when the caterpillars resulting from these moths might appear. Calculations on Monday, April 28, using the captures for the week of April 19-25, 2008 to represent the peak flight for the first generation this year, predict farmers can expect to see peak caterpillar activity for the first generation to occur around May 20 in Princeton and May 25 in Lexington.

“Certainly caterpillars will be present before this date because moths were flying before this last week,” he said. “The moths that flew the previous week should be present by May 17 in Princeton and May 23 in Lexington. Remember these are only estimates. The model uses 2008 temperatures for those dates that have passed, but uses the average temperature for the past five years, for future dates. Thus the estimates should get better when we use more data from 2008. Also, the traps are only sampled weekly so we can come no closer than plus or minus one week.”

Additionally, these dates are based on the Princeton and Lexington locations only. Other areas of the state that are warmer or cooler may have populations that appear earlier or later.

Some fields may need to be treated to control the insect. Scouting is the key to determining if a field will need to be treated. The trapping information along with information calculated by degree-day model can assist farmers in scouting their fields for armyworm infestations.

For more information on true armyworms and how to scout your fields for them and other pests, contact a county office of the UK Cooperative Extension Service.